1. Field of the Invention
The present invention relates to a composite magnetic head having a recording/playback head and an erasing head combined in one unit, and more particularly, to a composite magnetic head which is made to have a reduced distance between gaps of both magnetic heads while improving reduction of cross talk in a compact high-density recording and reproducing apparatus.
2. Description of the Background Art
FIGS. 4 and 5 illustrate a conventional composite magnetic head of bulk type. FIG. 4 is a plan view of the magnetic head and FIG. 5 is a sectional view taken along the line A--A of FIG. 4.
A recording/playback head 1 and an erasing head 2 are combined in one unit through a nonmagnetic plate 3 in order to avoid magnetic interaction therebetween. The recording/playback head 1 is constituted by a first core half 5 made of ferrite, for example, which is formed with a coil groove 4 by machining such as cutting and a second core half 6 made of ferrite, for example, which is arranged to be opposed to the first core half 5. Further included is a gap spacer 7 made of silicon dioxide, for example, which is interposed between the both core halves 5 and 6, reinforcing portions 8 made of glass or the like, for example, which are provided in the vicinity of the joint portion between the both core halves 5 and 6, and an energizing coil 9 which is wound in the coil groove 4 around the first core half 5.
On the other hand, the erasing head 2 is constituted by a first core half 11 made of ferrite, for example, which is formed with a coil groove 10 by machining such as cutting and a second core half 12 made of ferrite, for example, which is arranged to be opposed to the first core half 11. Further included is a gap spacer 13 made of silicon dioxide, for example, which is interposed between the both core halves 11 and 12, reinforcing portions 14 made of glass or the like, for example, which are provided in the vicinity of the joint portion between the both core halves 11 and 12, and an energizing coil 15 which is wound in the coil groove 10 around the first core half 11.
In FIG. 4, an arrow X indicates the travelling direction of a magnetic recording medium (not shown) such as a magnetic disc. The recording/playback head 1 is arranged on the upstream side of the erasing head 2 with respect to the travelling direction X. The magnetic gap of the recording/playback head 1 formed by gap spacer 7 and the magnetic gap of the erasing head 2 formed by the gap spacer 13 are arranged to satisfy the relationship therebetween as shown in FIG. 4. Therefore, the data recorded over a track of the magnetic disc by means of the recording/playback head is partially erased at its one or both side end portion(s) by means of the succeeding erasing head 2 to provide an accurately recorded track, or thereby assuring off-track margin(s) of the head.
FIGS. 6 and 7 are explanatory views of the manner of formation of the recorded track with using of this kind of composite magnetic head. FIG. 6 shows the manner of formation of the recorded track in the outer peripheral region of the magnetic disc and FIG. 7 shows the manner of formation of the recorded track in the inner peripheral region of the magnetic disc.
In these drawings, reference numerals 16, 17, 18, 19, 20 and 21 denote a magnetic gap of the recording/playback head, a magnetic qap of the erasing head, recorded tracks after the width-correction by the erasing head 17, a center of rotation of the magnetic disc, the erased width on the inner peripheral side to be erased by the erasing head, and the track pitch between adjacent tracks, respectively. The magnetic gap 16 of the recording/playback head 16 extends radially, while the gap 17 extends in parallel with the gap 16.
The magnetic disc is provided with annular recording zones having a predetermined width, and the curvatures of the recording zones are different from each other between the annular recording zones at the outer and inner peripheral regions thereof. Comparing the annular recording zone at the outer peripheral region having a smaller curvature with that at the inner peripheral region having a larger curvature, the erased radial width 20 to be erased by the erasing head is larger for the annular recording zone at the inner peripheral region (FIG. 7) than for the zone at the outer peripheral region (FIG. 6). As a consequence, the track pitch 21 shows a delicate change depending on the radial position of the tracks. This is notable particularly in a case where the curvature shows a great change between the radially outer and inner peripheral regions in a compact or small magnetic disc, and furthermore, in a case where the distance L between the magnetic gap 16 of the recording/playback head and the magnetic gap 17 of the erasing head 2 is long.
Difference in the track pitch (the distance between adjacent tracks) depending upon the radial position thereof mentioned above brings about difficulty particularly in the case of high-density recording.
In addition, the coercive force of the magnetic recording medium has recently become large as a result of increase in the recording density, so that it becomes impossible in some cases to overwrite sufficiently with use of the recording head. For this reason, it is proposed as well that recording is preceded by erasing. In this case as well, it is necessary to reduce the distance between the gaps of the both heads for the reason mentioned above.
To cope with the above-mentioned problems, there has been made attempts to thin the second core halves 6 and 12 by grinding/polishing so as to reduce the distance L between the magnetic gaps 16 and 17. However, in the case of mechanical cutting mentioned above, the gap distance L is limited to 0.8 mm or so in consideration of the manufacturing yield, so that it is impossible to obtain a satisfactory effect.
In order to provide a composite magnetic head which overcomes such disadvantages and in which the recording/playback head and the erasing head are combined in one unit and the distance between both magnetic gaps is reduced sufficiently so as to be suitably used in a compact high-density recording and reproducing apparatus, the applicants have proposed a composite magnetic head as disclosed in Japanese Patent Application No. 62-236293.
In the composite magnetic head described above, the recording/playback head and the erasing head are each coated with a nonmaqnetic thin layer and are joined with each other. Coating of the nonmagnetic layer makes it possible to thin a nonmagnetic layer serving to joint the recording/playback head and the erasing head with each other greatly, thereby making it possible to reduce the distance between the both magnetic gaps.
However, in the case where the erasing head has an insufficient erasing ability, there is caused a phenomenon called crosstalk which deteriorates the recording and reproducing characteristic of the composite recording head.
It is noted here that crosstalk is caused due to leakage of the magnetic flux of the erasing head to the recording/playback head at the time of recording and due to leakage of the magnetic flux induced from the gap of the erasing head to the recording/playback head at the time of reproduction. FIG. 8 shows this phenomenon, in which reference symbols .PHI..sub.0, .PHI..sub.1, .PHI..sub.2 and .PHI..sub.3 represent the magnetic flux passing through the core of the erasing head remote from the recording/playback head, the magnetic flux passing through a portion of the erasing head adjacent to the recording and reproducing head, the magnetic flux passing through a portion of the recording/playback head adjacent to the erasing head, and the magnetic flux passing through the core of the recording/playback head remote from the erasing head, respectively.
As seen from FIG. 8, the magnetic fluxes .PHI..sub.1, .PHI..sub.2, and .PHI..sub.3 are parallel to each other so that it is necessary to reduce the magnetic fluxes .PHI..sub.2, and .PHI..sub.3 in order to improve the recording and reproducing characteristic of the composite magnetic head.
To this end, it is necessary to reduce the magnetic reluctance of the core of the erasing head to a valve adjacent to the recording/playback head less than that of the core of the recording/playback head.
Heretofore, polycrystalline ferrite has been used as the material for the erasing head from the viewpoint of cost and the like since the magnetic reluctance has not been a serious problem in the erasing head as compared with the recording/playback head, resulting in the problem that the crosstalk cannot be decreased.